1. Field of the Invention
This invention relates to a method for producing a rare-earth element containing iron or iron and boron alloy, and particulary a dysprosium-iron-boron alloy, adapted for use in the manufacture of rare-earth element containing, iron-boron permanent magnets.
2. Description of the Prior Art
It is known to produce permanent magnet alloys of a light rare-earth element, such as neodymium, in combination with iron and boron. It has been determined that light rare-earth element containing magnets of this composition may be improved from the standpoint of increasing coercivity by incorporating therein the heavy rare-earth element, dysprosium. The amounts of dysprosium used for this purpose vary within the range of 0.5 to 8% by weight, depending upon the coercivity required.
Dysprosium is conventionally added to light rare-earth element containing iron-boron magnets by introducing dysprosium in elemental form prior to alloy melting.
To obtain dysprosium of a purity suitable for introducing to an alloy melt, high-cost refining practices are required, which add significantly to the overall cost of producing the alloy. Dysprosium oxide, however, is significantly less expensive than the pure element dysprosium.
It is known to alloy dysprosium with iron by a reduction-diffusion process embodying calcium as the reductant. The amount of metallic calcium used may vary from 1.2 to 3.5 times (weight ratio) the amount stoichiometrically necessary to reduce the oxygen in the dysprosium oxide. The alloy may also contain additional elements such as boron and other rare earth elements in minor amounts with iron and dysprosium being the major constituents of the alloy. It is also known to include calcium chloride (CaCl.sub.2) as an ingredient in the reduction-diffusion process for the purpose of aiding in particle disintegration during calcium oxide removal steps.
Thereafter, the alloy in particle form is mixed with a light rare earth element containing, iron-boron alloy in the desired proportions to achieve the final alloy composition. The powder mixture is processed conventionally to produce permanent magnets which includes cold pressing, sintering, and heat treatment.
In the reduction-diffusion process, calcium oxide (CaO) results as a by-product from the calcium reduction of the dysprosium oxide (Dy.sub.2 O.sub.3). Prior to further processing and use of the dysprosium-iron-boron alloy, it is necessary to remove the calcium oxide, as well as any excess, unreacted calcium.
This is achieved by washing with water which reacts with the calcium and calcium oxide to produce calcium hydroxide (Ca(OH).sub.2). These reactions are exothermic: EQU Ca+2H.sub.2 O.fwdarw.Ca(OH).sub.2 +H.sub.2 +heat (99.2 Kcal/mole) EQU CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2 +heat (15.6 Kcal/mole).
Consequently, the particle size of the comminuted reaction mass must be maintained rather large (8 mesh U.S. Standard) so that the surface area available for reaction is small and heat is generated at a slow and manageable rate. Smaller particle sizes and larger reaction areas result in sudden exothermic heating causing water temperatures to approach the boiling point. This is undesirable since the reduced rare earth metals may readily be re-oxidized.
The calcium chloride interspersed within the 8 mesh particles is more soluble in water than the other constituents. This allows the particles to slowly decrepitate as the calcium chloride is dissolved. It also creates new calcium and calcium oxide reaction surfaces at a rate where their heat generation is manageable. An undesirable aspect of including calcium chloride is that compounds such as dysprosium chloride (DyCl.sub.3) or iron chloride (FeCl.sub.3) may be formed during the reduction-diffusion step. Such compounds are also very water soluble and are thereby lost with the wash water. This adds to the overall cost of the process by reducing the amount of usable alloy recovered.
The particle size of the final washed material should be on the order of 35 mesh or finer so that it may expeditiously be further comminuted to 2 to 3 micron powder for the purpose of magnet manufacturing.